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Nilsen-Moe A, Rosichini A, Glover SD, Hammarström L. Concerted and Stepwise Proton-Coupled Electron Transfer for Tryptophan-Derivative Oxidation with Water as the Primary Proton Acceptor: Clarifying a Controversy. J Am Chem Soc 2022; 144:7308-7319. [PMID: 35416654 PMCID: PMC9052761 DOI: 10.1021/jacs.2c00371] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Concerted electron-proton transfer (CEPT) reactions avoid charged intermediates and may be energetically favorable for redox and radical-transfer reactions in natural and synthetic systems. Tryptophan (W) often partakes in radical-transfer chains in nature but has been proposed to only undergo sequential electron transfer followed by proton transfer when water is the primary proton acceptor. Nevertheless, our group has shown that oxidation of freely solvated tyrosine and W often exhibit weakly pH-dependent proton-coupled electron transfer (PCET) rate constants with moderate kinetic isotope effects (KIE ≈ 2-5), which could be associated with a CEPT mechanism. These results and conclusions have been questioned. Here, we present PCET rate constants for W derivatives with oxidized Ru- and Zn-porphyrin photosensitizers, extracted from laser flash-quench studies. Alternative quenching/photo-oxidation methods were used to avoid complications of previous studies, and both the amine and carboxylic acid groups of W were protected to make the indole the only deprotonable group. With a suitably tuned oxidant strength, we found an ET-limited reaction at pH < 4 and weakly pH-dependent rates at pH > ∼5 that are intrinsic to the PCET of the indole group with water (H2O) as the proton acceptor. The observed rate constants are up to more than 100 times higher than those measured for initial electron transfer, excluding the electron-first mechanism. Instead, the reaction can be attributed to CEPT. These conclusions are important for our view of CEPT in water and of PCET-mediated radical reactions with solvent-exposed tryptophan in natural systems.
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Affiliation(s)
- Astrid Nilsen-Moe
- Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 523, 75120 Uppsala, Sweden
| | - Andrea Rosichini
- Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 523, 75120 Uppsala, Sweden
| | - Starla D Glover
- Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 523, 75120 Uppsala, Sweden
| | - Leif Hammarström
- Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 523, 75120 Uppsala, Sweden
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Benseghir Y, Solé-Daura A, Mialane P, Marrot J, Dalecky L, Béchu S, Frégnaux M, Gomez-Mingot M, Fontecave M, Mellot-Draznieks C, Dolbecq A. Understanding the Photocatalytic Reduction of CO2 with Heterometallic Molybdenum(V) Phosphate Polyoxometalates in Aqueous Media. ACS Catal 2021. [DOI: 10.1021/acscatal.1c04530] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Affiliation(s)
- Youven Benseghir
- Université Paris-Saclay, UVSQ, CNRS, Institut Lavoisier de Versailles, 78000 Versailles, France
- Laboratoire de Chimie des Processus Biologiques, UMR CNRS 8229, Collège de France, Sorbonne Université, PSL Research University, 75231 Paris Cedex 05, France
| | - Albert Solé-Daura
- Laboratoire de Chimie des Processus Biologiques, UMR CNRS 8229, Collège de France, Sorbonne Université, PSL Research University, 75231 Paris Cedex 05, France
| | - Pierre Mialane
- Université Paris-Saclay, UVSQ, CNRS, Institut Lavoisier de Versailles, 78000 Versailles, France
| | - Jérôme Marrot
- Université Paris-Saclay, UVSQ, CNRS, Institut Lavoisier de Versailles, 78000 Versailles, France
| | - Lauren Dalecky
- Université Paris-Saclay, UVSQ, CNRS, Institut Lavoisier de Versailles, 78000 Versailles, France
| | - Solène Béchu
- Université Paris-Saclay, UVSQ, CNRS, Institut Lavoisier de Versailles, 78000 Versailles, France
| | - Mathieu Frégnaux
- Université Paris-Saclay, UVSQ, CNRS, Institut Lavoisier de Versailles, 78000 Versailles, France
| | - Maria Gomez-Mingot
- Laboratoire de Chimie des Processus Biologiques, UMR CNRS 8229, Collège de France, Sorbonne Université, PSL Research University, 75231 Paris Cedex 05, France
| | - Marc Fontecave
- Laboratoire de Chimie des Processus Biologiques, UMR CNRS 8229, Collège de France, Sorbonne Université, PSL Research University, 75231 Paris Cedex 05, France
| | - Caroline Mellot-Draznieks
- Laboratoire de Chimie des Processus Biologiques, UMR CNRS 8229, Collège de France, Sorbonne Université, PSL Research University, 75231 Paris Cedex 05, France
| | - Anne Dolbecq
- Université Paris-Saclay, UVSQ, CNRS, Institut Lavoisier de Versailles, 78000 Versailles, France
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Neumann S, Wenger OS, Kerzig C. Controlling Spin-Correlated Radical Pairs with Donor-Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis. Chemistry 2021; 27:4115-4123. [PMID: 33274791 PMCID: PMC7986886 DOI: 10.1002/chem.202004638] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2020] [Revised: 12/02/2020] [Indexed: 12/30/2022]
Abstract
One-electron reduced metal complexes derived from photoactive ruthenium or iridium complexes are important intermediates for substrate activation steps in photoredox catalysis and for the photocatalytic generation of solar fuels. However, owing to the heavy atom effect, direct photochemical pathways to these key intermediates suffer from intrinsic efficiency problems resulting from rapid geminate recombination of radical pairs within the so-called solvent cage. In this study, we prepared and investigated molecular dyads capable of producing reduced metal complexes via an indirect pathway relying on a sequence of energy and electron transfer processes between a Ru complex and a covalently connected anthracene moiety. Our test reaction to establish the proof-of-concept is the photochemical reduction of ruthenium(tris)bipyridine by the ascorbate dianion as sacrificial donor in aqueous solution. The photochemical key step in the Ru-anthracene dyads is the reduction of a purely organic (anthracene) triplet excited state by the ascorbate dianion, yielding a spin-correlated radical pair whose (unproductive) recombination is strongly spin-forbidden. By carrying out detailed laser flash photolysis investigations, we provide clear evidence for the indirect reduced metal complex generation mechanism and show that this pathway can outperform the conventional direct metal complex photoreduction. The further optimization of our approach involving relatively simple molecular dyads might result in novel photocatalysts that convert substrates with unprecedented quantum yields.
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Affiliation(s)
- Svenja Neumann
- Department of ChemistryUniversity of BaselSt. Johanns-Ring 194056BaselSwitzerland
| | - Oliver S. Wenger
- Department of ChemistryUniversity of BaselSt. Johanns-Ring 194056BaselSwitzerland
| | - Christoph Kerzig
- Department of ChemistryUniversity of BaselSt. Johanns-Ring 194056BaselSwitzerland
- Department of ChemistryJohannes Gutenberg University MainzDuesbergweg 10—1455128MainzGermany
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